Milling tools for chip removing machining of, above all, workpieces of metal (steel, aluminum, titanium, etc.) are generally composed of a rotatable basic body or milling-cutter body, most often of steel, as well as a plurality of replaceable milling inserts of cemented carbide, ceramics or the like. Because the milling inserts are expendable materials as a consequence of being worn fairly fast, it is most often desirable to form the same with as large a number of cutting edges as possible. For this reason, the milling inserts may be carried out double-sided so far that the underside is formed with the same number of cutting edges as the upperside, while doubling the number of cutting edges in comparison with single-sided milling inserts. Therefore, milling cutters for face milling are often equipped with milling inserts that are double-sided and have a quadratic basic shape with four cutting edges, i.e., four pairs of co-operating main edges and secondary edges, along the upperside as well as the underside, and which are mounted at an effective setting angle of approximately 45° in the milling-cutter body. In such cases, the main edge and the secondary edge form a nominal angle of 135° with each other.
The problems forming the basis of the present invention are associated with double-sided face milling inserts of the type that is denominated “negative,” and which are formed with clearance surfaces that extend perpendicularly to the neutral plane to which the uppersides and undersides are parallel. In order to provide requisite clearance between, on one hand, the clearance surface present (rotationally) behind the active, surface-wiping secondary edge, and on the other hand the generated, plane surface of the workpiece, the milling insert has to be mounted with a negative axial tipping-in angle in the milling-cutter body. Simultaneously, the milling insert also has to have a negative radial tipping-in angle in order to provide clearance between, on one hand, the clearance surface behind the chip-removing main edge, and on the other hand the generally cone-shaped surface generated by the same. Just the negative axial tipping in of the milling insert gives, on one hand, rise to greater axial cutting forces than those cutting forces that arise when the milling inserts are positively tipped-in, and on the other hand problems with the chip formation as well as the chip evacuation difficult to master, among others so far that the chips tend to be directed obliquely downward toward the generated plane surface rather than clear from the same.
Before the background of the invention is further described, it should be pointed out that certain fundamental concepts found in this document, e.g., “clearance angle,” can be of either a nominal or an effective character. When, for instance, a clearance angle is “nominal,” the same only relates to the milling insert as such, i.e., without conjunction with the milling-cutter body, but if the same is “effective,” reference is made to the clearance angle that occurs when the milling insert is mounted in the rotatable milling-cutter body and performs chip removal.
The problems caused by the negative axial and radial, respectively, tipping in are particularly accentuated in milling cutters having the older type of milling inserts, which have a prismatic basic shape and include cutting edges, the main edges of which are straight and pair-wise parallel along common clearance surfaces, as well as also the secondary edges are straight and pair-wise parallel along the common clearance surfaces thereof. In this case, the main edges will be subjected to particularly great cutting forces and give rise to considerable chip formation and chip evacuation problems, because the milling inserts have to be tipped into an equally great negative axial angle, as the desired effective clearance angle between the generated plane surface of the piece to be machined and the clearance surface behind the surface-wiping secondary edge.
More recently, a number of proposal of solutions of the above-mentioned problems have appeared. Thus, in U.S. Pat. No. 5,807,031, a double-sided, quadratic face milling insert is disclosed, the chip-removing main edges of which are inclined in relation to the neutral plane of the milling insert, more precisely in such a way that the individual main edge, counted from a first end adjacent to the co-operating secondary edge, first declines toward the underside of the milling insert and then, from a lowest point, again rises toward an opposite end. In such a way, the above-mentioned problems are solved in a general way, so far that the effective axial angle of the proper main edge is reduced from a relatively great negative value to a smaller, more positive value, in spite of the milling insert, per se, (i.e., the neutral plane of the milling insert) has a sufficiently great negative axial angle in order to provide the requisite clearance behind the surface-wiping secondary edge. However, this known milling insert is, nevertheless, associated with a number of shortcomings and disadvantages. One such disadvantage is that the two secondary edges along a common clearance surface of each corner of the milling insert are still straight and mutually parallel. This means that the transition between the individual secondary edge and the appurtenant main edge forms a fairly sharp corner (as viewed nominally in side view), so far that the angle between the secondary edge and the declining main edge is considerably smaller than 180°. Thus, in the preferred embodiment, this angle amounts to 165°-170°. Because the corner transition between the main edge and the secondary edge is the part of the milling insert being absolutely most exposed to, among other things, forces, heat and erosion, the fairly sharp corner of the same means that the milling insert becomes fragile and gets a limited service life associated with the wear thereof. In addition, the wear of such a sharp corner easily gives rise to visible stripes in the finished, wiped-off surface, more precisely in the form of shallow, per se, but nevertheless most detrimental grooves in the surface being plane in other respects. In other words, the finish of the generated surface becomes rather mediocre. Another disadvantage is that the peripheral borderline, which surrounds the upperside (and the underside, respectively), is a single continuous cutting-edge line. Thus, the cutting-edge line of the individual main edge transforms directly into an adjacent, non-co-operating secondary edge—or alternatively via diminutive part edges along facet surfaces between the clearance surfaces of the main edge and the secondary edge—the secondary edges forming the uppermost portions of the upperside, i.e., no other points along the upperside are situated at a greater distance from the neutral plane than the secondary edges. This is destructive considering that just the secondary edges are crucial for the finish of the generated surface, and therefore should be sharp or in any case undamaged as long as possible. Because the secondary edges in the known milling insert stick up in relation to the rest of the upperside, the same are subject to miscellaneous risks of damage. Thus, damage may easily arise during the handling of the milling inserts, e.g., when the same are placed on metal tables or the like in connection with indexing and replacements. Because each active main edge directly transforms into an adjacent, inactive secondary edge, there is, in addition, a risk that the removed chips hammer against and damage the secondary edge not yet used. Moreover, the declining or downwardly leaning part of the main edge, which extends from the co-operating secondary edge toward the lowermost point of the main edge, will have a length that is considerably greater than half the length of the main edge. This means that the material in the milling insert becomes considerably thinner in the area of the lowermost point or depression of the main edge, whereby the strength of the milling insert is impaired. This detriment will be particularly marked when the milling insert requires large clearances.
A face milling insert that resembles the milling insert described above and which is essentially impaired with the same disadvantages as the same, is previously described in U.S. Pat. No. 7,306,409.
The present invention aims at obviating the above-mentioned disadvantages of the previously known milling inserts and at providing an improved milling insert. Therefore, an object of the invention to provide a milling insert, and in particular a double-sided face milling insert, the most sensitive parts of which, i.e., the corner transitions between co-operating secondary and main edges are strong and well functioning, in spite of the fact that the main edges of the milling insert can work with effective rake angles that are moderately negative or even positive in spite of the effective clearances behind the secondary edges being ample.
Another object of the invention is to provide a milling insert, in which the risk of damage to the sensitive secondary edges is reduced to a minimum.
Yet another object of the invention is to provide a milling insert, in which the desired, cutting-technical improvements can be attained without the milling insert being weakened by unnecessary reduction of the amount of material (cemented carbide) in the same.
Still another object of the invention is to provide a double-sided face milling insert particularly suitable for fine milling or semi-fine milling, the main edges of which initially decline at a greater angle than the remaining parts of the same, in order to, in such a way, additionally refine the qualities of the milling insert in connection with fine milling, i.e., milling with small or moderate cutting depths.